ECG of the Week – 10th July 2019 – Interpretation

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You are reviewing a 43 yr old hypertensive patient who has been referred by their GP after presenting with central chest pain (currently pain free.) They have provided previous ECGs from today and a month before.

ECG from a month prior
ECG from today

Interpretation:

Electrode reversal (LA/LL)

  • Leads I and II switched
  • Lead III upside down (may not appear abnormal)
  • Leads aVF and aVL switched
  • P wave unexpectedly larger in lead I than lead II (although not seen in this ECG)
  • As explanation I and aVL are now inferior, II and aVF are now lateral

Electrode Reversal in General:

  • 10 electrodes (RA/RL/LA/LL/V1-6) produce a 12 lead ECG
  • Limb electrodes contribute to leads APART from RL which acts to eliminate artefact.
  • Leads I,II,II are obtained by combining the output of two limb electrodes.
  • aVL, aVF, aVR compare a single lead electrode by comparing to a ‘common’ electrode that is the sum of the two remaining limb electrodes.
  • Chest electrodes also use a ‘common’ electrode for comparison by combining the three limb electrodes (RA,LA,LL) this means if limb electrodes are reversed it will not affect chest leads.

When to suspect Limb Electrode Reversal:

  • QRS axis is unexpectedly abnormal
  • P waves are inverted in limb leads, especially lead I and II
  • aVR is upright and another leads resembles aVR
  • leads I,II,III are virtually a flat line

When to suspect Chest Electrode Reversal:

  • normal P, QRS,T progression across the precordium is interrupted

Management:

as appropriate for chest pain without ECG changes, when suspecting ACS:

  • Control pain – GTN, Morphine (although this patient is pain free)
  • Aspirin 300mg
  • +- Ticagrelor / Heparin as per local policy / level of suspicion
  • Blds inc Trop
  • CXR

Textbook (upon which I heavily relied):

  • Chan TC, Brady WJ, Harrigan RA, Ornato JP, Rosen P. ECG in Emergency Medicine and Acute Care. Elsevier Mosby 2005.